I’m a bit disappointed about the details given. The explanation given by Markus Vilcinskas on TechNet forums: What’s New in FIM update 1 has some interesting extra information:

A new prerequisite for Update 1: The FIM Synchronization Service is now using SQL Native Agent, so be sure to install it before you install the update

FIM Synchronization Service changes:

Added support for Recycle Bin in Active Directory. To get the correct result, install the Active Directory Recycle Bin fix 979214 on the DC used by FIM Synchronization Service.

Added a new feature called “Resume Full Sync”. If a full sync is not completed the next time the same run profile is attempted a new option in Run Management Agent will allow the administrator to continue the Full Sync from the point that it stopped.

Addressed an issue to detect Exchange Server 2010 in the AD MA.

In my opinion the update is really advised, especially if you’re using Exchange 2010 and/or Active Directory based on Windows 2008 R2